This invention relates to ultrasonic thickness and acoustic velocity measuring instruments. More specifically, this invention refers to an ultrasonic measuring instrument for determining the thickness of a workpiece, the instrument including automatic zeroing (calibrating) means, automatic temperature compensation and high temperature warning means, automatic identification means of the probe, means for correcting the measurement readings for probe characteristics, means for storing and displaying of the minimum wall thickness during a predetermined time interval, and means for providing a display indicative of the difference between a measured thickness and a preprogrammed value.
It is well known in pulse-echo ultrasonic thickness measuring that ultrasonic energy transmitted into a workpiece is reflected at a defect or other acoustic discontinuity, such as the entrant surface or rear wall. By measuring the time of travel of the ultrasonic energy signal through a workpiece of known acoustic velocity, i.e. from the time the ultrasonic energy enters the workpiece until a rear wall echo signal is received, the thickness of the workpiece can be determined. Conversely, if the thickness of a workpiece is known, the acoustic velocity can be determined.
Some ultrasonic probes, particularly those used in corrosion testing of plates or pipes, are dual-element probes, also referred to as "pitch-catch" probes. These probes are used because of the rough rear surfaces encountered during corrosion testing.
The piezoelectric transducer elements associated with dual probes typically are mounted on blocks of heat insulating coupling means, also referred to as delay lines, which are usually constructed of polymeric thermoplastic material. The probe elements, coupled to respective delay lines, are inclined at a predetermined angle with respect to the entrant surface of a workpiece. The transmit element is coupled to a first delay line and the receive element is coupled to a second delay line, both delay lines being of the same material. The two delay lines are electrically and acoustically isolated by an acoustic barrier, such as cork, and the entire assembly is enclosed in a probe housing.
In the present invention, the transmit element, which also acts as a receive element, is mounted on a first delay line which is longer than the delay line upon which the receive element is mounted. In addition, the delay line coupled to the transmit element contains an artificial reflector, for instance, a transversely drilled notch. The notch is located at a distance "X" from the front face of the delay line. The delay line upon which the receive element is mounted is dimensioned to be shorter than the first delay line by a distance "2X." The transmit/receive element measures the time for an ultrasonic signal transmitted by the transmit element to reach the notch whereat the signal is reflected and returns to the same element. This measured time is used to identify a specific probe type. If the delay line is characterized by a length L, the round trip distance traveled by the notch reflected ultrasonic signal in the delay line is 2(L-X).
After determining the probe type used by means of the notch reflected echo signal, preprogrammed probe characteristic factors are retrieved from a PROM of a microprocessor contained in the instrument. The calibration factors include correction for path length errors due to the angle of incidence of the transmit signal and echo signal. Retrieval of the proper values is assured since the measured notch reflected echo signal is different for each probe type. The use of preprogrammed memory for providing probe compensation data in an ultrasonic pulse-echo instrument is described, for example, in U.S. Pat. No. 4,102,205, issued to W. Pies et al., entitled "Method and Apparatus for Ultrasonic Nondestructive Testing of Workpieces with Automatic Compensation for the Probe, Workpiece Material, and Temperature," dated July 25, 1978.
Having identified the probe type used, the instrument is calibrated by coupling a dual-element probe to a workpiece of known thickness T. The path length of a rear wall reflected ultrasonic signal transmitted from the transmit element through the delay line and workpiece to the rear wall and back to the receive probe is 2L-2X+2T. By subtracting the transit time of the notch reflected signal from the transit time of the rear wall reflected signal, and adjusting for the angle of incidence of the ultrasonic energy signals, the instrument will be calibrated by virtue of an evaluation unit calculating a required offset value in order to display the known thickness T. In addition to being calibrated, the instrument is thereafter automatically temperature compensated for variations of the transit time of signals traveling through the delay lines due to thermal effects. Moreover, if the notch reflected signal transit time changes by more than a predetermined value, the probe is becoming too hot. The instrument provides a warning of this condition to an operator prior to the probe being damaged. In prior U.S. Pat. No. 4,182,155 issued to K. A. Fowler, entitled "Method and Apparatus for Zero Point Calibration of Ultrasonic Thickness Gauge" dated Jan. 8, 1980, a dual-element probe having delay lines of different lengths for calibrating an ultrasonic thickness gauge is described. In the Fowler patent, prior adjustment of the instrument using a gauge block is required for calibration whereas in the present invention calibration is performed automatically by the instrument.
Another improvement of the present invention resides in the automatic activation feature. A calibration specimen is provided on the face of the instrument. Coupled to the calibration specimen is a receive probe. When the instrument is in the off condition, periodic monitoring of the receive probe is performed. The microprocessor contains C-MOS circuitry which when the instrument is in the off condition operates at a low pulse repetition frequency for minimizing the power consumption. If the receive probe detects several ultrasonic signals passing through the calibration block, the unit is activated.
When the instrument is activated, the probe type is automatically identified, and the microprocessor calculates a zeroing offset value for causing the readout to display the known thickness of the calibration block. Both the thickness and the acoustic velocity of the specimen are known and permanently stored in the instrument memory. The calculated offset signal zeroizes the instrument and is used in all subsequent measurements. This means of calibrating the instrument is an alternative to the use of an external workpiece of known thickness and acoustic velocity for calibration.
In addition, in a store mode the instrument using either hardware or software, retains the minimum thickness measurement display for a predetermined time interval, usually for several seconds. In thickness testing of corroded workpieces, while the probe scans the workpiece surface, the minimum thickness measurement is often calculated and displayed faster than the operator can make an observation. Storage and display of the minimum thickness for several seconds allows the operator to observe the minimum reading measured during the time interval and note the area tested. The operator can then rescan the workpiece to find the exact location of the minimum reading or perform other corrective action to the workpiece.
The instrument also is programmed to display a difference thickness reading. That is, instead of displaying the true measured thickness, the readout will display the difference between the measured signal and a predetermined nominal value. This feature is particularly useful in quality control applications.
Further and still other provisions of the present invention will become more clearly apparent when the following specification is read in conjunction with the accompanying drawings.